Microphones used in automotive electronic applications, such as cell phones, navigational systems, and vehicular control, are well-known in the art. An automotive vehicle presents challenges to the use of a microphone in view of the numerous sources of noise that can interfere with vocalized speech inside the vehicle. These challenges can be particularly difficult when adapting a microphone solution for use in the vehicular rearview mirror assembly. In addition to the difficulties of rejecting noise within the vehicle, disturbances to the sound field caused by the rearview mirror must also be addressed.
The prior art includes systems that use microphones positioned in tandem, i.e., a first microphone positioned in front of a second microphone. This type of system works to produce a difference signal for canceling noise by subtracting the signals and using a delay to account for the distance between the microphones. However, the rearview mirror disturbs the sound field between the two microphones, which results in poor subtraction over much of the frequency range of interest. Additionally, this front and back microphone configuration requires the rearview mirror to include a deeper housing for supporting the rearward microphone, which is often an undesired design feature in view of styling, weight, vibration sensitivity, and molding required in the manufacturing process.
Other prior art systems use microphones positioned in parallel that use digital processing or simple delay networks to improve operation. The use of digital processing introduces delay and variation over time that disrupts systems designed for a single microphone. Therefore, this type of simple delay based processing does not yield the desired performance. Additionally, many of the microphone systems currently in use were developed under the assumption that the microphone would be used in connection with a handheld cellular phone. In handheld applications, the very close proximity of the user's mouth to the microphone assures a very high speech-to-noise content for most situations. These systems do not function correctly with microphones used at a distance because audio received at increased distances does not exhibit the same frequency characteristics.
Microphones distant from an audio source that are used in a hands-free car system, unlike a very close use situation, will often have a very significant noise content, and manifest a wider dynamic range. A “close use” situation or microphone may be defined as one positioned within 20 cm of the audio source such as a user's mouth. The dynamic range is increased because of the broader range of possible speech signal levels and relative noise content. In a distant use situation, if a wider dynamic range speech signal is processed via the phone system, especially phones employing code division multiple access (CDMA), much of the desired speech can be lost because the processing system is unable to correctly determine that speech is present. Thus, the phone system functions as if a voice signal is comprised of only noise.
The current state of the art seeks to lower the noise content while retaining the speech in its unaltered state. This process does not restore the nature of the speech signal to that of a close use microphone as found in a typical handset and as a result does not yield a signal able to pass through the cell phone's CODEC. As a result, there will be many frequency bands or occurrences where the speech content, though significant, is not great enough to overcome the residual noise to the extent so as to avoid being interpreted as noise. Thus, in latter processing stages, these frequency bands or occurrences will be removed because they appear to be only unwanted noise. Even though the speech content is significant, it is not of a great enough magnitude to overcome the noise in certain frequency bands or at certain times.
Moreover, most cars driven above 50 mph, on rough roads, will have less than acceptable speech quality through the cellular phone when using a Bluetooth connected hands-free microphone system because of limited dynamic range. These problems are magnified where the cellular phone is a CDMA type because the CODECs employed in these phones are less tolerant of a wide dynamic range signal. The CODEC system, in attempting to limit band width, stops correctly transmitting the speech signal because it interprets it as being unwanted noise. In some cases, the speech components in the cellular call can be totally lost. The loss can be such that the user may feel they have suffered a classic cellular phone drop out when, in fact, the call is still in progress and connected. Since there are literally millions of cellular phones with CODECs implementing a bandwidth/noise reduction algorithm that will truncate speech as described, the only hope for a solution is a process that will result in a signal whose speech content will be passed even when noise is also present.
There are three aspects that can address incorrect CODEC operation, processing of the microphone signal to emulate the signal from a close used microphone, reduction of the noise proportion in a way that does not otherwise harm system operation, and an elevation of all significant content speech frequency bands to a magnitude well above all other bands. In every case the threshold where an action is invoked is can be variable based on a number of factors including the average noise level, peak noise levels, least noise level, average speech levels, least speech level and peak speech levels. The intent being to process the speech carrying signal to minimize the impact of any noise present. This processing ideally needs to be adjusted to align with the conditions present at that time.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.